1. Field of the Invention
The present invention is generally directed to the analysis of well log data, and, more particularly, to the removal of periodic errors from well log data.
2. Description of the Related Art
Generally, well bores are not uniform throughout their length, but have a variety of irregularities that are generally referred to as borehole rugosity. Borehole rugosity may be caused by a number of factors. For example, some methods of drilling can produce boreholes that are slightly spiral, i.e., the rotation of the drill pipe and/or downhole drilling motor can produce a borehole having different diameters throughout its length. When a logging tool, such as an array induction tool, gamma ray tool, neutron tool, etc., is positioned in the borehole, the rugosity of the borehole changes the amount of standoff of the tool at different locations along the length of the borehole. These differences in standoff can produce periodic errors in the well log data obtained by the tool.
Often, periodic errors in well log data due to borehole rugosity are treated by well log analysts as interference since the periodic errors do not represent the particular formation under consideration, nor do they represent a normal tool reading that would be expected in a non-rugose borehole. Moreover, in some cases, periodic errors due to the rugosity of the borehole can completely smear a well log, e.g., a borehole having a corkscrew rugosity where there is a strong sinusoidal corkscrew signal coupled with its harmonics.
One technique proposed for removing sinusoidal noise due to borehole rugosity is described in U.S. Pat. No. 5,579,248, entitled "Method for Removing Borehole Rugosity Noise From Well Log Data." The technique disclosed therein involves the steps of detrending the well data, saving the non-stationary trend information of the data, calculating a Fourier amplitude spectrum of the residual series, identifying rugosity peaks in the amplitude spectrum of the residual series, applying a single frequency notch filter to the residual series, and adding the filtered residual series back to the saved trended data.
However, the technique disclosed in U.S. Pat. No. 5,579,248 has several shortcomings. In particular, the method described therein involves the use of a complicated cubic splines procedure to detrend and restore the data, it does not disclose a method for automatic identification of the periodic errors due to borehole rugosity on a frequency spectrum, it does not disclose a technique for estimation of the frequency of periodic errors due to borehole rugosity, and only a single frequency notch filter is proposed to remove periodic errors. Such shortcomings can result in, among other things, increased time and computing requirements for determining and removing the sinusoidal noise from the well log data.
The present invention is directed to a method and apparatus that solves or reduces some or all of the aforementioned problems.